The present invention relates to the field of ambulatory and non-invasive monitoring of an individual""s physiological parameters. In particular, the invention relates to an apparatus and method for extracting a cardiac signal from a signal generated by a thoracocardiograph (TCG) that may also contain respiratory and motion/noise signals.
As used herein, xe2x80x9cplethysmographyxe2x80x9d, and its derivative words, is the measurement of a cross-sectional area of a body. xe2x80x9cInductive plethysmographyxe2x80x9d is a plethysmographic measurement based on determination of an inductance or a mutual inductance. A xe2x80x9cplethysmographic signalxe2x80x9d is a signal generated by plethysmography, and specifically by inductive plethysmography. The cross-sectional area of the body measured by a plethysmograph, also referred to herein as a thoracocardiograph (TCG), may include, singly or in combination, the chest, abdomen, neck, or arm.
The inductance sensor may be as simple as a conductive loop wrapped around the body cross-section. The loop is attached to a close-fitting garment that expands and contracts with the body cross-section. As the body cross-section expands and contracts, the area enclosed by the loop also expands and contracts thereby changing the inductance of the loop. The inductance change of the loop may be converted to an electrical signal using methods known to one of skill in the electrical art.
If the loop is placed around the chest, the changes in the loop inductance may be correlated to respiration volumes. For example, U.S. Pat. No. 4,308,872 (xe2x80x9c""872 patentxe2x80x9d), issued Jan. 5, 1982 and titled xe2x80x9cMethod and Apparatus for Monitoring Respiration,xe2x80x9d discloses a method and apparatus for monitoring respiration volumes by measuring variations in the patient""s chest cross sectional area and is herein incorporated by reference in its entirety.
In addition to measuring respiration volumes, a plethysmograph may also measure cardiac volumes and aortic pulses as described in U.S. Pat. No. 5,178,151 (xe2x80x9c""151 patentxe2x80x9d), issued Jan. 12, 1993 and titled xe2x80x9cSystem for Non-invasive Detection of Changes of Cardiac Volumes and Aortic Pulses,xe2x80x9d and herein incorporated by reference in its entirety.
U.S. Pat. No. 6,047,203 (xe2x80x9c""203 patentxe2x80x9d), issued Apr. 4, 2000 and titled xe2x80x9cPhysiologic Signs Feedback System,xe2x80x9d discloses a non-invasive physiologic signs monitoring device which includes a garment that may be worn and has a plurality of sensors disposed on the garment such that respiratory and cardiac signs may be measured and transmitted to a remote device. The ""203 patent is herein incorporated by reference in its entirety.
Co-pending U.S. patent application Ser. No. 09/836,384 (xe2x80x9c""384 applicationxe2x80x9d), filed on Apr. 17, 2001 and titled xe2x80x9cSystems and Methods for Ambulatory Monitoring of Physiological Parameters,xe2x80x9d discloses a system and method for non-invasive, ambulatory monitoring of pulmonary and cardiac parameters and is herein incorporated by reference in its entirety.
The plethysmographic, or TCG, signal generated by the inductance sensor placed around the chest will be composed of essentially three signals generated from different sources. The first, and largest component of the TCG signal is caused by respiration and has a characteristic frequency that varies from about 12 breaths per minute to about 30 breaths per minute. The second, and smaller, component of the TCG signal is generated by the expansion and contraction of the heart within the chest cavity and is characterized by a frequency that varies from about 50 beats per minute to about 100 beats per minute (or more) in the resting state. The third component of the TCG signal is caused by motion or noise and cannot be characterized by a narrow range of frequencies. In order to extract cardiac parameters from the TCG signal, the cardiac component must be separated from the respiratory and noise components of the TCG signal. Although no further mention of the noise component of the TCG signal will be made, when referring to the respiratory, or pulmonary, component of the TCG signal, it should be understood to include the noise or motion component of the TCG signal as well.
Separating the cardiac signal from the pulmonary signal in the plethysmograph signal is difficult, if not impossible, for two reasons. First, the cardiac and pulmonary signals are composite signals having component frequencies close to each other (for example, 0.8-1.7 Hz cardiac frequency, 0.2-0.5 Hz pulmonary frequency) making frequency separation of the signals difficult. Moreover, the harmonics of the component frequencies of the respiratory signal lie directly within the spectrum defining the cardiac signal thereby making the complete separation of the cardiac signal from the respiratory signal impossible. Complete separation of the cardiac and respiratory signals, however, is not required for cardiac parameter extraction but will affect the resolution and accuracy of the extracted cardiac parameter. Furthermore, the frequencies of both the cardiac and pulmonary signals may change at different rates depending on the physical exertion of the subject. Second, the relative amplitude of the cardiac signal may be approximately 20 times smaller than the pulmonary signal and can vary by as much as a factor of three depending on the level of physical exertion thereby requiring very efficient removal of the pulmonary signal in order to recover the cardiac signal.
Two methods for separating the cardiac signal from the pulmonary signal are disclosed in the ""151 patent. The first method takes cardiac measurements only during breath-holding thereby eliminating the pulmonary contribution to the plethysmograph signal. Breath-holding is intrusive, however, and may cause discomfort to the subject. The second method averages the plethysmograph signal based on an external trigger signal associated with a cardiac event such as the R wave of an EKG or the upstroke of a systemic arterial pulse. The disadvantage of the average method is the loss of fine details due to the averaging.
Therefore, there remains a need for more efficient signal processing of the plethysmograph signal and extraction of the cardiac signal.
Citation or identification of any references in this Section or any section of this Application shall not be construed that such reference is available as prior art to the present invention.
One aspect of the present invention is directed to a method for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter, the method comprising the steps of: performing a frequency domain filtering operation on the plethysmographic signal producing a first filtered signal; performing a time domain filtering operation on the first filtered signal, producing a second filtered signal; and extracting the cardiac parameter from the second filtered signal. The frequency domain filtering operation may include a band-pass filter and furthermore be characterized by a lower corner frequency that is determined by a heart rate.
Another aspect of the present invention is directed to a method for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter, the method comprising the steps of: performing a frequency domain filtering operation on the plethysmographic signal producing a first filtered signal; performing a time domain filtering operation on the first filtered signal, producing a second filtered signal; and extracting the cardiac parameter from the second filtered signal wherein the time domain filtering operation that includes an ensemble averaging operation.
The ensemble averaging operation further comprises the steps of: associating a plurality of segments of the plethysmographic signal with events characteristic of a cardiac cycle; shifting a plurality of segments to align the events associated with each of the plurality of events characteristic of the cardiac cycle; constructing an ensemble averaged cardiac cycle signal from the average of the plurality of aligned segments. The event characteristic of a cardiac cycle comprises an indicia derived from the electrocardiographic R-wave. The ensemble averaging operation further includes the step of reconstructing a thoracocardiograph signal from the ensemble averaged cardiac cycle signal.
Another aspect of the present invention is directed to a method for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter wherein the cardiac parameter is a stroke volume.
Another aspect of the present invention is directed to a method for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter wherein the cardiac parameter is a cardiac output.
Another aspect of the present invention is directed to a method for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter wherein the cardiac parameter is a pre-ejection period.
Another aspect of the present invention is directed to a method for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter wherein the cardiac parameter is a peak ejection rate.
Another aspect of the present invention is directed to a method for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter wherein the cardiac parameter is the time to peak ejection rate.
Another aspect of the present invention is directed to a system for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter, the system comprising: a first frequency-domain filter receiving the plethysmographic signal, having a dynamic lower cutoff frequency, and producing a first-filtered signal, a second time-domain filter receiving the first filtered signal and producing a second filtered plethysmographic signal; and a processor for extracting the cardiac parameter from the second filtered signal.
Another aspect of the present invention is directed to a system for generating a thoracocardiograph signal comprising: a first digitizer for converting a first signal generated by an inductive plethysmographic sensor to a digitized first signal; a first digital filter for transforming the digitized first signal into a first filtered signal, the first filter characterized by a frequency pass-band based on a heart rate; and a second digital filter for transforming the first filtered signal into a thoracocardiograph signal, the second filter characterized by averaging segments of the first filtered signal based on events characteristic of the cardiac cycles.
Another aspect of the present invention is directed to a computer-readable medium comprising instructions for controlling a computer to generate a thoracocardiograph signal from a plethysmographic signal responsive to cardiac activity by frequency domain filtering the plethysmographic signal producing a first filtered signal; and time domain filtering the first filtered signal producing thoracocardiograph signal.
Another aspect of the present invention is directed to a method for extracting cardiac parameters from a plethysmographic signal characterized by a heart rate, the method comprising the steps of: performing a first band-pass filtering operation on the plethysmographic signal producing a first filtered signal, the first filtering operation characterized by a lower corner frequency less than the heart rate; performing a second band-pass filtering operation on the plethysmographic signal producing a second filtered signal, the second filtering operation characterized by a lower corner frequency greater than the lower corner frequency of the first filtering operation; interpolating the first filtered signal and the second filtered signal based on the heart rate to produce a filtered plethysmographic signal; and extracting cardiac parameters from the filtered plethysmographic signal.
Another aspect of the present invention is directed to a system for extracting cardiac parameters from a plethysmographic signal comprising: means for receiving a heart rate; a first filter characterized by a first lower corner frequency, the first lower corner frequency not greater than the heart rate, the first filter capable of receiving the plethysmographic signal and generating a first filtered signal; a second filter characterized by a second lower corner frequency, the second lower corner frequency greater than the first lower corner frequency, the second filter capable of receiving the plethysmographic signal and generating a second filtered signal; and a processor for generating a filtered plethysmographic signal by interpolating the first filtered signal and the second filtered signal based on the heart rate and extracting a cardiac parameter from the filtered plethysmographic signal.